The purpose of this investigation is to define the biochemical mechanisms of cardiac inotropic responses to hormones and drugs which have their actions mediated by cyclic AMP or calcium. The basic approach will be directed toward elucidation of the physiological role of protein phosphorylation by cyclic AMP-dependent protein kinase and a calcium dependent protein kinase (identified as phosphorylase kinase). Both kinases catalyze the phosphorylation of cardiac troponin and myosin light chains, regulatory proteins of the actomyosin contractile system. Thus, these two enzymes may have dual roles in promoting glycogenolysis and increasing cardiac contractility in response to stimuli related primarily to beta-adrenergic receptor function. The relationship between protein-bound phosphate of troponin and myosin light chains, and inotropic state will be evaluated in intact cardiac muscle preparations after stimulation by catecholamines, ions and appropriate pharmacological agents. Phosphoprotein formation will also be correlated to the metabolic changes induced by the hormones and drugs to define conditions that suggest possible biochemical mechanisms of regulation. These phospho- and dephosphoproteins will be examined for differences in biochemical properties, both as purifed proteins and in interactions with other proteins of the actomyosin complex in order to determine a possible biochemical mechanism for altered physiological performance of cardiac muscle. Other phosphoproteins that may be important for regulation of cardiac contractility will be identified, and differences between the phosphorylated and dephosphorylated species will be examined in vivo as well as in cell free systems. BIBLIOGRAPHIC REFERENCE: Buss, J.E. and Stull, J.T. Calcium binding to cardiac troponin and the effect of cyclic AMP-dependent protein kinase. FEBS Letters 73, 107, 1977.